A determination device for a passive safety system is proposed in JP-A-11-194137. In the passive safety system, an airbag is inflated in two steps using a satellite sensor and a floor G sensor. The satellite sensor is disposed at the front part of a vehicle and the floor G sensor is disposed in an electronic control unit ECU that is located under the instrument panel above the floor tunnel. The airbag system includes two inflators. The determination device detects the difference in phase between two acceleration waveforms obtained from the satellite sensor and the floor G sensor. Then, the determination device determines the severity of collision based on the detected phase difference. Both of two inflators in the airbag system are actuated if the severity is high, that is, the accident is serious. The airbag is inflated at a high pressure when both inflators are actuated and therefore an impact of the collision on the occupant is reduced.
The occupants may receive excessive impact from the airbag inflated at the high pressure if the severity is low, that is, the accident is not serious. When the severity is determined as low, only one of the two inflators is actuated to inflate the airbag at a low pressure. As a result, the impact of the airbag on the occupant is reduced.
However, the determination device of the proposed passive safety system requires two kinds of sensors: a satellite sensor and a floor G sensor. If a malfunction occurs in one of the sensors, the severity may not be properly determined.